In the binary form of continuous ink jet printing ink is directed, under pressure, through an array of orifices (formed in an orifice plate) to produce a plurality of ink jet filaments directed toward a print zone. The orifice plate is stimulated (e.g. by vibration) to regulate the break-up of the filaments into droplet streams. The stimulation ensures that each of the drops formed from a given filament break-off at essentially the same phase relative to the plate vibration or stimulation source. While some stimulation means, such as U.S. Pat. No. 4,683,477, are intended to produce substantially the same break-off phase for all jets in the array, the break-off phase varies significantly from jet to jet with traveling wave stimulation such as is described in U.S. Pat. No. 3,739,393.
Drop charge electrodes are located adjacent the drop break-off regions of respective filaments, and when energized with a voltage, induce a charge of opposite polarity on the drops that are then breaking off the filament ends. The energization of drop charge electrodes is controlled by cyclic gating of groups of "on" or "off" information signals to electrode drivers. Typically, charged drops are deflected to a catcher device and uncharged drops pass on to the print surface.
The charge electrode driving electronics is designed for a normally biased condition, i.e. normally catch drop producing. To produce a print drop on the charge electrode, voltage must be dropped to near zero volts for an interval which includes the drop break-off. For drop generators, such as U.S. Pat. No. 4,683,477, which produce substantially uniform drop break-off phases for all jets in the array, the print pulses are applied (as needed) at a common phase relative to the stimulation source. By properly phasing the print pulse, using one of many known procedures for determining the proper phase, drops from all the jets in the array can be properly controlled.
Drop generators employing traveling wave stimulation produce drop break-offs at essentially all phase angles relative to the stimulation source. Print pulses which are at constant phase relative to the stimulation source are likely to produce bands of print defects parallel to paper motion. The bands of defects correspond to drops breaking off with phases outside the print pulse or with phases corresponding to the transient leading and trailing edges of the print pulse. While the print pulse width can be increased to 360.degree. wide to ensure that no jets have drop break-off outside the print pulse, there is always a possibility of jets with drops breaking off during the pulse transients. Even though the charging voltage is changing very rapidly during the transient, drops with break-off during the transient are charged in various amounts from near zero charge (corresponding to the print drops) up to the charge of the catch drops. The partially charged drops may be caught if the drop charge is high, or they may strike the print media as an improperly deflected print drop.
Thus, a problem with prior art traveling wave stimulation printers (with their non-synchronous drop break-off) is that they have several jets along the array with drops breaking-off in the switching interval. Rather than try to eliminate such switching period drop errors, the prior art printers have used designs which randomize these "switching" errors. For example, by clocking the phase of the address cycles only from a print medium tachometer signal, with no reference to the phase of traveling wave stimulation, there is produced a randomization of the location of the switching period drop defects. By randomizing the defect locations, defects may be less objectional than if located in one or more bands across the print.
While helpful to some extent, the prior art randomizing approach still results in many stray dots on the print medium. Also, the number of drops per pixel printed will not be uniform. This produces banding in large area prints, especially half tones, as well as dot size variation in printed dot matrix characters.
Many long array, plane wave drop generators also will produce substantial drop break-off phase variation across the array. With such drop generators the large break-off phase variation can make it impossible or very difficult to choose a print pulse phase which does not produce some switching transient related print defects.